An optical fiber used for a transmission path of light in an optical communications system comprises a core for transmitting light, and a clad layer formed around the core and having a smaller refractive index than that of the core. As optical switches for switching optical paths of a plurality of such optical fibers, many types of optical switches have conventionally been proposed. As a mechanical switch, JP 63-313111 A proposes, as shown in FIG. 28, an optical-fiber-movable, 1×2-type optical switch having a structure in which a magnetic layer 5 attached to an input optical fiber 1A is directly driven by a magnetic coil 4 to switch an optical path 3 of an input optical fiber 1A from an output optical fiber 1B to an output optical fiber 1C. The optical fibers 1A, 1B, 1C are fixed to holders 6A and 6B of a pipe 16, and the magnetic layer 5 of the input optical fiber 1A is held at a predetermined position in the pipe 16 by a magnetic power of a permanent magnet 7 mounted onto an outer surface of the magnetic coil 4.
JP 54-162551 A and JP 55-87107 A disclose, as shown in FIGS. 29 and 30, a micro-optical-element-movable, 1×2-type optical switch for switching optical paths by moving a micro-optical element such as a mirror, prism, etc.
In the optical switch shown in FIG. 29, optical fibers 1A, 1B, 1C are disposed on a circle at an interval of 120° with a reflection surface of a reflection mirror 2 as a center. The input optical fiber 1A is provided with an optical system (not shown) for turning a light beam to a parallel beam. A parallel light beam from the input optical fiber 1A impinges on a reflection mirror 2 along an optical path 3, reflected thereby and enters into an output optical fiber 1B as an output light along an optical path 3a. When the reflection mirror 2 rotates by 120° C., a parallel light beam reflected by the reflection mirror 2 enters into the output optical fiber 1C as an output light along an optical path 3b. Thus, by controlling the rotation angle of the reflection mirror 2, the output optical fibers 1B, 1C can be selected.
In the conventional micro-optical-element-movable switch shown in FIG. 29, however, when there is a large rotation angle of the reflection mirror 2, the optical switch needs a large area because the optical fibers are radially arranged.
In the optical switch shown in FIG. 30, output optical fibers 1B, 1C, 1D, 1E, 1F, 1G are arranged on a circle with an input optical fiber 1A as a center. A parallel light beam from the input optical fiber 1A is reflected by two 45°-reflection mirrors 2 along an optical path 3, and enters into an output optical fiber 1B along an optical path 4. By controlling the rotation angle θ of the reflection mirror 2, each output optical fiber can be selected.
The rotation angle and position of a prism or a mirror is controlled by a pulse motor, etc. in a conventional micro-optical-element-movable switch. However, because positioning precision on the sub-micron order is needed in the coupling of optical fibers used in optical communications, position control by a pulse motor, etc., whose rotation angle precision is as low as about 0.5°, provides only poor reproducibility in the switching of optical paths, resulting in increase in loss. Also, it is disadvantageous in that current should always be supplied to a motor.
In the conventional optical switch shown in FIGS. 2(a) and (b), the rotation angle θ of a reflection mirror 2 is used to distribute a light beam from 1A to 1B and 1C. A light beam from 1A to 1B impinges on and is reflected by a reflection mirror 2 at an angle φ1 relative to a normal D1 of the reflection mirror 2, while a light beam from 1A to 1C impinges on and is reflected by a reflection mirror 2 at an angle φ2 relative to a normal D2 of the reflection mirror 2. φ1 and φ2 are half of the rotation angle θ of the reflection mirror 2, φ1=φ2=θ/2.
An actual input optical fiber is provided with a lens for turning a light beam to a parallel light beam, and light reflected by the reflection mirror 2 should enter into a lens of an output fiber at center. Accordingly, the optical fibers should be separated from each other by a distance D equal to or higher than the radius of the lens. Thus, the distance LB, LC from the reflection mirror 2 to each output optical fiber 1B, 1C is LB=LC=D/tan φ1. If the rotation angle θ is too small, the distance LA, LB, LC between the reflection mirror 2 and the optical fibers 1A, 1B, 1C should be long enough to prevent their mutual interference, resulting in a larger optical switch. Also, in the case of a conventional optical switch as shown in FIG. 2(a), in which optical paths are switched only by the rotation of a reflection mirror 2, a reflection mirror 2 is always moving, necessitating the control of the rotation angle θ with high precision.
FIG. 4 shows the calculation results of the distance L between the reflection mirror 2 and the optical fibers relative to the distance D between the optical fibers when the rotation angle θ of the reflection mirror 2 is set at 4°, 6°, 8° and 10°, respectively. As is clear from FIG. 4, when the rotation angle θ of the reflection mirror 2 is small, the distance L between the reflection mirror 2 and the optical fiber should be long. Because a collimator lens used has a diameter of about 3.2 mm, the distance D should be 2 mm or more taking into account the diameter of a light beam. Therefore, the distance between the reflection mirror 2 and the optical fiber is about 28 mm at a rotation angle θ of 8°, resulting in a larger optical switch. In this case, because an optical path length is as long as 56 mm from the input optical fiber 1A to the output optical fibers 1B, 1C, there is large loss in coupling to the optical fibers, resulting in difficulty to achieve low loss.
With respect to optical fibers, too, there is a problem of reflection return. The reflection return is a phenomenon that occurs by the difference in a refractive index between the core and air when light transmitting in a core of an optical fiber reaches its end. Because a reflection return light returns to a light-emitting element such as a semiconductor laser, etc., an oscillation state becomes unstable. Accordingly, to suppress the reflection return light, the end of an optical fiber is cut slantingly or rounded, or a reflection-preventing film is formed on the end of the optical fiber to alleviate the difference in a refractive index between the core and air.
It has been common that light exiting from one optical fiber is collimated by a lens and caused to impinge on the other optical fiber. Used as a lens for this purpose are a GRIN lens of a refractive index distribution type, etc. However, the GRIN lens is extremely larger than the optical fiber, resulting in a larger apparatus.
JP 54-20747 A discloses the high-efficient coupling of light to a light-emitting element or a light-receiving element by rounding the ends of optical fibers. As shown in FIG. 31, when a spherical portion 105 is simply formed at a tip end of an optical fiber 100, the radius R of curvature of the spherical portion 105 can be made larger than the radius (D1/2) of the optical fiber, though a spherical surface cannot be enlarged at the end of a core 106. Accordingly, light 103 exits from the end of the optical fiber substantially in the same manner as before the tip end is turned to a spherical portion 105, failing to achieve high coupling to a light element, etc. The term “high coupling” means that an optical fiber and a light element or both optical fibers are coupled at high efficiency while suppressing the leak of light. In addition, there is substantially the same level of a reflection return light as before a spherical portion 105 is provided at a tip end of the optical fiber.
As a method for treating the end of an optical fiber, JP 1-269906 A discloses a method of melting a core 106 and a clad layer 107 uniformly, and forming its tip end to a semicircular shape. FIG. 32 shows an optical fiber core 106 having a spherical tip portion 105 formed by melting a core 106 and a clad layer 107. In this optical fiber, the distance T from the tip end of the spherical tip portion 105 to the core is smaller than the diameter (2R) of the spherical tip portion 105. Accordingly, the expanding angle NA of light 103 emitted from the spherical tip portion 105 is substantially the same as the expanding angle of light in the core. As a result, a light beam 103 exiting from the spherical tip portion 105 so expands that the spherical tip portion 105 cannot act as a collimator.